Field
The invention relates generally to wireless communication, and more specifically to data rate control in a wireless communication system.
Background
Wireless communication systems are used in many applications including, for example, paging, wireless local loops (WLL), Internet telephony, wireless telephone and satellite communication systems. An exemplary application of a wireless telephone system is a cellular telephone system for remote subscribers which are often mobile. In a typical cellular telephone system mobile subscribers, or mobile stations, transmit and receive signals from various base stations within a wireless network infrastructure of the communication system as the mobile station moves about.
Modern wireless communication systems, such as cellular telephone system, are typically designed to allow multiple users, or subscribers, to access a common communications medium. Various techniques have been developed for these multiple access wireless communication systems including code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA). These multiple access techniques encode, modulate, decode, and demodulate signals transmitted and received between the multiple users and the wireless network infrastructure, thereby enabling simultaneous communication among multiple users and allowing for a relatively large capacity for the communication systems.
In a wireless communication system based on CDMA, the available radio frequency (RF) spectrum is shared efficiently among a number of users. Wireless communication systems typically transmit voice messages and more recently, systems with enhanced capacity for data services are also available. An example of such a data-services communication system is the high data rate (HDR) system that conforms to the Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA) cdma2000 High Data Rate Air Interface Specification IS-856, January 2002 (the IS-856 standard).
In a wireless communication system, such as one based on CDMA or one of the other multiple access techniques mentioned, users are often mobile. As a user moves about they may move out of the coverage area of a sector of a base station or out of the coverage area of the base station itself. As a user, also referred to as a mobile or a mobile station, moves in and out of different coverage areas such that the user leaves one coverage area and enters another, to maintain communications a technique known as a “handoff” occurs. In a handoff the mobile station begins communicating with a sector of a base station, simply referred to as a base station, in the coverage area it is entering and terminates communication with the base station in the coverage area it is leaving. Using a technique referred to as “soft handoff” the mobile station will simultaneously be in communication with the two base stations during the handoff. In other words, the mobile will remain in communication with the base station whose coverage area the mobile is leaving while also establishing communication with the base station whose coverage area the mobile is entering. In such a technique, both the base stations either jointly or independently decode the mobile stations transmission. Communicating with both base stations during the soft handoff reduces the chance of a dropped call or other unintentional loss of communication.
The data rate that can be supported by each of the two base stations involved in a handoff may be different, for example, due to the level of congestion of the respective base station. The level of congestion in a system may be determined by monitoring the data rates of users, and the received signal strength required to achieve a desired quality of service (QoS). The communication link from the mobile to the base station is called the reverse link or uplink. In a wireless CDMA system, the reverse link capacity is interference-limited and one measure of the cell congestion is the total received power over the level of the thermal noise at a base station. The total received power over the thermal noise is commonly referred to as the “rise over thermal” (ROT) and corresponds to loading of the reverse link. Typically, it is desirable to maintain the ROT near a predetermined value. If the ROT is too high, the coverage of the cell, that is the distance over which the base station of the cell can communicate, is reduced and the reverse link is less stable. A reduced cell coverage (such as from excessive ROT) can adversely affect the data rate that can be supported in the cell and the mobile stations at the edge of the cell may drop their calls. The coverage of the cell reduces with high ROT due to an increase in the amount of transmit energy required at the mobile station to provide a target energy level at the base station. Typically mobiles are somewhat limited in the transmit power they have and thus a requirement to increase transmit power corresponds to a decreased range. A low ROT can indicate that the reverse link is not heavily loaded, thus indicating that available data rate capacity is potentially being wasted.
If a base station and a mobile involved in a handoff can support different data rates, then the data rate of the mobile station during a handoff may not be optimal. For example, if the base station to which the mobile is handing off can support a higher data rate than is being used by the mobile, then the mobile may be operating at a lower data rate than is possible, in which case there may be a waste of system resources. If the base station to which the mobile is handing off cannot support a data rate as high as the mobile is operating with, then the mobile can be causing increased interference with other users and could be degrading system performance. A form of rate management in a soft-handoff region can be achieved with co-ordination between base-stations. However, the co-ordination between base-stations over the infrastructure, or backhaul, may be slow or there may not be any support to carry out the co-ordination between two base stations.
There is therefore a need in the art for improving distributed data rate control during handoff in a wireless communication system.